Stable oscillator



March 14, 1961 R H. DlCKE 2,975,376

STABLE OSCILLATOR Fld April 50, 1958 amr f/ .bf/Ayi Mawr) ff) Aff-,rr :ggf/gf l Fl 2. INVENTOR. ju RUBEN H. DICKE United States Patent O STABLE OSCILLATOR Robert H. Dicke, 37 Jefferson Road, Princeton, NJ.

Filed Apr. 30, 1958, Ser. No. 732,057

7 Claims. (Cl. 331-94) The present invention relates to a stable oscillator and particularly to an improved, self-oscillating atomic clock.

The invention is an improvement on the one described in application, Serial No. 542,420, filed October 24, 1955, by the applicant and Thomas R. Carver which issued on May 27, 1958, as Patent No. 2,836,722. In the latter, the oscillator includes an atomic or molecular resonance apparatus and there is continuous feedback from the output to the input of the apparatus to sustain oscillations. ln the present invention, the self-regeneration occurs only as a result of the long memory of a molecular resonance and, even with a very weak gas resonance, the oscillations remain locked to the frequency of the molecular resonance.

According to a preferred form of the invention, a pulse is applied to an electromagnetic wave resonant substance at a resonant frequency of the substance. The substance produces oscillations during the pulse interval and, after the pulse has ceased, continues to ring for a time which is long compared to the pulse interval. A portion of the ringing is employed to produce the next exciting pulse for the substance.

In a specific form of the invention, a signal from a local oscillator is mixed with a signal from the resonant substance to produce a beat note. The beat note is am plied and delayed and then mixed with the local oscillator signal. An exciting pulse of the resultant signal, a component of which is at the resonant frequency, is applied to the resonant substance. The substance oscillates during the exciting pulse and, upon the termination of the pulse, continues to ring. This cycle is continuously repeated, the delay introduced in the feedback loop being such that the pulse applied to sustain oscillations is always the sum of the ringing oscillation and the local oscillation. Preferably, the pulse time is relatively short compared to the ringing time. As an example, the pulse time may be l microsecond and the ringing time l() microseconds. Accordingly, the oscillator system is free running the major portion of the time, whereby the output is determined mainly by the atomic or molecular resonant frequency of the substance and is extremely stable.

The invention will be described in greater detail by reference to the following description taken in connection with the accompanying drawing in which:

Fig. l is a block circuit diagram of a preferred form of the invention; and

Fig. 2 is a drawing of waveforms present at various places in the circuit of Fig. 1.

Block 10 in Fig. 1 titled a Transmission type atomic or molecular resonance apparatus" is described in detail in the application referred to above. In brief, block 10 may include a vessel filled with an alkali earth metal vapor such as sodium, potassium, rubidium or the like, mixed with a buifer gas. The alkali earth metal vapor may be placed in its excited condition, that is, in a condition such that an applied microwave at the resonant frequency of the vapor can cause it to oscillate, by optical pumping. '111e vessel containing the gas may be en- Patented Mar. 14, 1961 ICC closed in a cavity resonator, waveguide or the like. An arrangement of this general type is shown in Figs. 1 and 2 of the aforegoing application and described in detail therein. Alternatively, the vessel containing the gas may be out in the open and the microwaves for exciting the vapor applied by means of a horn or other electromagnetic wave beam forming means.

in other forms of the invention, block 10 may be an arrangement such as shown in Fig. 3 of the above application and explained in detail therein. In brief, this comprises a two chambered diffusion arrangement which includes two cavity resonators having a common aperlured wall. Gas molecules diffuse between the two resonators. The gas may be ammonia, ethyl chlorate or one of the other gases listed in the application. When properly excited, the gas in the arrangement can be made to oscillate.

One may assume that noise or some other transient effect has caused some oscillations at a resonant frequency to be produced by the resonant substance in circuit 10. 'lhese are applied via the output lead 12 to a mixer 14. Oscillations generated in local oscillator stage 16 are applied to the mixer via arms 18 and 20 of the magic T, hybrid junction 22. The local oscillator 16 may be a c klystron, magnetron, or the like and its frequency should be close to that of the resonant frequency produced by circuit 10. To permit the use of standard components in the stages following mixer 14, the local oscillator frequency may differ from the resonant frequency by 30 megacycles, for example.

Local oscillations are also applied via arms 18 and 24 of the magic T to a modulator stage 26. The fourth arm of the magic T is preferably matched to prevent reiections back to the magic T junction. This is illustrated schematically by symbol 28.

Returning to the right of the figure, the output of mixer 14, which consists of a 30 megacycle intermediate frequency signal, is applied to amplifier 28, and the amplitied intermediate frequency signal goes to the delay line 3h. The delay line introduces a delay which is equal to or greater than the duration of a pulse produced in stage 34. The output of the delay line 30 is applied through a gate 32 to the modulator 26. The gate is opened and closed by means of short pulses applied from puiser 34. The gate (switch) closed time is preferably only a small fraction of the gate (switch) open time, say, 1A@ or so. As a result, the delayed intermediate frequency signal output of stage 32 consists of relatively short pulses. 'l' he modulator' 26 mixes the intermediatefrequency, pulse-modulated signal from stage 32 with the local oscillations from stage 16. The modulator output at lead 36 consists of pulses at the resonant frequency of the resonant substance in circuit 10. The delays in the system are such that the pulses are regeneratively applied to circuit 10 and sustain the oscillations therein. Upon the termination of the pulse, the resonant substance continues to ring.

The operation of the system can be better understood by referring to Fig. 2. The continuous output of local oscillator 16 is shown at b. Itis at a frequency fo which is 3() megacycles from the resonant frequency of the resonant substance in circuit 10. The output of pulser 34 is shown in Fig. 2a. The pulses from modulator 26 are shown in Fig. 2c. Assuming that the pulse 38 is the first one applied to the electromagnetic wave resonant substance, the output of the substance consists of the oscillations as shown in Fig. 2d. During the excitation inierval, the substance oscillates and thereafter continues to ring. There is some decay in the output of circuit 10 during the ringing. This is exaggerated somewhat in Fig. 2d. The ringing oscillations are at a precise, resonant frequency of the electromagnetic wave resonant substance. The wave shown in Fig. 2d is mixed with the continuous wave output of local oscillator 16 to produce an intermediate frequency wave. The latter is delayed by delay line 30 as shown in Fig. 2e. The delay 1- should be greater than the duration of pulse 38. Accordingly, thenext time stage 32 is gated; the signal which passes through this stage-consists only of the beat note between the ringing oscillations from circuit 10 andV the local oscillations vfrom stagey 16. This is the wave shown in Fig. 2f and it is at the intermediate frequency of 30 megacycles. This pulse (f) is mixed'withthe local oscillations shown in Fig. 2b-toproduce a sum frequency pulse signal at lead 36; The sum frequencyis the precise resonant frequency of the electromagnetic wave resonantfsubstancc.

Pulses at the resonant' frequency of the gas are available at lead 36, the output' connection being shown at 40, ContinuousA waves at the intermediate frequency may be taken at the output of amplifier stage 28, the output connection being shownY at 42. This signal may be detected and a D.C; produced which is indicative of any change in the frequency output of the circuit 10. The DC. may, if desired; be fed back to the local oscillator 16 to stabilize the oscillator.

What isY claimed is:

1. A stable oscillator comprising, in combination, an excited electromagnetic wave resonant substance which is resonant at a given electromagnetic wave frequency; means for applying a first pulse of electromagnetic waves at said frequency to said substance, whereby the latter produces oscillations at said frequency and, after the termination of said iirst pulse, rings for a given interval of time; and means responsive only to a portion of the ringing oscillations for rcgeneratively applying a later pulse than said first pulse of electromagnetic waves at the ringing frequency to the electromagnetic wave resonam substance so as to sustain oscillations therein.

2. Ai stable oscillator comprising, in combination, an electromagnetic wave resonant substance which is capable of producing an electromagnetic wave output at a resonant frequency thereof; means for applying a iirst pulse of electromagnetic waves at said frequency to said substance, whereby the latter produces oscillations at said frequency and, after the termination of said first pulse, rings at said frequency for a given interval of time; and means responsive only to a portion of the ringing oscillations for regeneratively applying a later pulse than said iirst pulse of electromagnetic waves at the ringing freqnency to said substance so as to sustain oscillations therein.

3. A stable oscillator comprising, in combination, an alkali earth vapor which is capable of producing microwave oscillations at a resonant frequency thereof; means operatively associated with said vapor for placing the va por in condition to oscillate at said frequency; means for applying a first pulse at said frequency to said vapor, whereby the latter oscillates during the pulse interval and, thereafter, continues to ring for a predetermined interval of time; and means responsive only to said ringing during said predetermined' interva for producing a later pulse than said first pulse at said frequency andA means for regeneratively applying said later pulse to said vapor during a successive pulse interval.

4. In the combination asset forth in claim 3, said means for placing said vapor in condition to oscillate comprising means for optically pumping said vapor.

5. A stable oscillator comprising, in combination, an electromagnetic wave resonant substance which is capable of oscillating at a resonant frequency; means including a local oscillator and a source of intermediate frequency pulses for producing sumfrequency pulses at said resonant frequency and` applying'the latter to said substance, wherebyV the substance oscillatesV during the pulse interval and thenV continues to oscillate for a given time interval; means for miningy thelocal oscillations with the osciltations produced by said substance to produce an intermediate frequency Wave; means for delaying the latter for an interval at least' equalV to that of the one of said pulses; and meansfor converting the delayed intermediate frequency wave to said intermediate frequency pulses, said last-named means comprising said source of intermediate frequency pulses.

6. A stable oscillatorV as set forth in claim 5, wherein said resonant substance comprises an alkali metal vapor.

7. A stable oscillator as set forth in claim 5, wherein said= source which provides intermediate frequency pulses provides pulses which are short compared to the interval after the termination of a pulse during which the substance continues to oscillate.

Enfer-emes Elited. in the file of this patent UNITED STATES PATENTS 2,417,834 Lord Mar. 25, 1947 2,697,172 Szerlp Dec. 14, 1954 2,762,871 Dicke Sept. 1l, 1956 2,795,775 Faymoreau June 11, 1957 2,836,722 Dicke et al May 27, 1958 2,851,604 Clapper Sept. 9, 1958 

